1. Field of the Invention
The present invention relates to a roller supporting or a general-purpose bearing unit for supporting a main shaft of a machining tool, etc.
2. Description of the Prior Art
Heretofore, a pillow block has been used as a supporting unit for a rotating shaft or a hollow roll and other rotating body in a rotatable or swingable manner. The structure of such a pillow block is shown in FIG. 25 in which a rotating body R.sub.1 is supported with 2 pillow blocks P at both ends under a bearing B.sub.1. In the figure, H shows a housing.
Another bearing unit, known in the prior art, is shown in the specification of U.S. Pat. No. 4,114,960 of Helmut Habermann et al. and FIG. 27 in which a magnetic bearing B.sub.2 supports magnetically a rotor shaft of a motor. According to this system, the position of a rotating shaft R.sub.2 is detected by a radial sensor RS and an axial sensor AS, while signals of detection controlling currents flowing in a radial electromagnetic pole RE and an axial electromagnetic pole AE, respectively, for controlling an attracting force of each electrode. Thus, the rotating shaft is supported rotatably in a non-contact state at a predetermined position.
With said pillow block P, a vibration system comprising a rotating body R.sub.1 --a bearing B.sub.1 --a housing H, is constituted. In this vibration system, a damping force in the system greatly depends on a damping force of the bearing. However, the damping force of a roller bearing is normally extremely small. Therefore if there is a large unbalance in the rotating body R.sub.1, a large centrifugal force occurs during revolution due to balance. This centrifugal force largely swings the entire pillow block via the bearing B.sub.1 of the pillow block P. As a result, a base on which the pillow block P is mounted is vibrated.
In addition, the rotating body R.sub.1 supported by the pillow block P is borne through a principal axis of shape m-m as shown in FIG. 26. However, the position of this rotating center axis cannot be changed freely. For example, the body R cannot be supported through the principal axis center of inertia n-n.
On the other hand, said magnetic bearing B.sub.2 is structured so that the rotating body R.sub.2 is floated in a completely non-contact state. However, where a long and large roll must be supported at both ends, mounting errors might become so large that a current for controlling the balance of the rotating body becomes large enough to pose economic disadvantage.
Another system of bearing, known in the prior art for supporting a rotor which is rotating at a high revolutional frequency against a stationary member, in a contactless manner in a radial direction, is disclosed in the U.S. Pat. No. 3,877,761 for example.
Furthermore, another system of a bearing known in the prior art is disclosed in the U.S. Pat. No. 4,683,391 in which an actuator comprises a rotor having teeth with a predetermined pitch and a revolution control coil wound on a plurality of pole pieces formed on a core member in opposition to said rotor.
These 2 preceding examples are structured in such a manner that the rotating shaft is supported contactless by magnetic control. However, similar to the example shown in FIG. 27 and the U.S. Pat. No. 4,114,960, a current for controlling the balance of the rotating body becomes so high that it is disadvantageous economically where a large roll must be supported at both ends and the error in mounting the shaft becomes excessive.
Also in this case, if mounting errors occur between the shaft and a magnetic pole during assembly work, the shaft or the pole might be damaged because of the interference between them. The heavier or longer the shaft is, the more severe this trend of damage results. In fact, assembly work become impossible.
In addition, where the weight of a load applied to the shaft becomes larger, a larger deflection load is created. As a result, a current supplied to the magnetic poles must be increased disadvantageously.